Combustion chambers fall into two categories: constant pressure and pressure rise. In a constant pressure combustor fuel at steady state is continually combusted and the hot exhaust gas allowed to expand without constraint. While there may be some pressure loss or pressure fluctuations due to resonance within the chamber these variations are kept small. Examples of constant pressure combustors are: turbo-annular combustors for gas turbines, ram jets or dump combustors. Such combustors are not said to be thermo acoustic.
In a pressure rise combustor the pressure within the combustor varies widely and in a periodic manner. A pressure rise combustor utilises unsteady combustion to produce an exhaust gas stream which has a higher mechanical energy, or stagnation pressure, than that of the inlet stream. The produced mechanical energy can be extracted as, for example, thrust or shaft work. Pressure rise combustors may be further divided into thermo acoustic combustors, where at least the outlet of the combustion chamber is open to atmosphere and the acoustics of the combustor are such that deflagration of the fuel/air mixture acts against an induced pressure wave to further increase the pressure in the chamber, and mechanical combustors in which the fuel/air mixture is constrained within an enclosed combustor and deflagration or detonation of the fuel/air mixture acts against a piston or other mechanical device. Pressure rise combustors may be used to provide propulsion. An example of a thermo-acoustic pressure rise combustor is a pulse-jet.
A pressure rise combustion system may be applied to a gas turbine and offers a potentially increased thermodynamic performance. In a conventional gas turbine combustion chamber, i.e. non pressure rise, there is a pressure loss of typically 5% of the engine overall pressure ratio and there is no conversion of thermal energy to mechanical energy.
In a pressure-rise combustor there is a stagnation pressure rise due to the conversion of chemical energy into mechanical energy. Pressure-rise combustion can be used with solid, liquid or gaseous fuels.
A pulse-jet may be valved or valve-less and utilise unsteady combustion in an acoustically resonant combustion chamber to produce a pressure rise. Fuel is steadily supplied to the combustion chamber and the timing of the unsteady combustion heat release is dictated by the aero-thermo-acoustical coupling in the working fluid. The phase angle and amplitude of the unsteady heat release is governed by the internal fluid mechanics of the system i.e. temporal and spatial variations of mixing processes, strain rates, convection of reactants and ignition sources. The combustion chambers burn the fuel in a deflagration process rather by detonation. The deflagration enables the combustion process to be self-sustaining in that once initial ignition is effected the acoustics within the system generate a cyclical combustion process without requiring further energy input to re-ignite an injected air/fuel mixture. This is in contrast to detonation combustors where an air/fuel mixture is detonated through input from an external energy source such as a spark plug, the chamber evacuated of the products of the detonation, a new air/fuel mixture supplied to the chamber and detonated through input from the external energy source. Each combustion event can be said to be isolated from an earlier and subsequent combustion event and consequently significant energy must be input by the spark plugs to ensure operation of the combustor for a sustained period.